The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well-known and is, in fact, the method used currently for commercial production of ethylene oxide. The typical catalyst for such purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain a porous, inert support or carrier upon which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. in Catalyst Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
It is also well-known, however, that the catalysts and reaction conditions which are best suited for ethylene oxide production do not give comparable selectivities in the direct oxidation of higher olefins such as propylene. The discovery of vapor phase direct oxidation processes capable of providing propylene oxide in higher selectivity than is presently attainable thus would be most desirable.